Method of producing silver layer on non-metallic electrically non-conductive support



United States Patent Claims. (Cl. 204-38) The invention relates to a method of manufacturing external, electrically conductive noble-metal layers on non-metallic, electrically non-conductive, at least superficially hydrophilized supports and to products manufactured by this method.

The term noble-metal layers is to be understood to mean herein not only layers having an uninterrupted surface of noble metal but also those having patterns, the portions of which may be discontinuous, for example, station-name dials for radio apparatus, ornamental objects, wiring patterns, circuit patterns, and so on.

According to the invention an external, electrically conductive noble-metal layer is produced on a non-metallic, electrically non-conductive, slightly hydrophilic or at least superficially hydrophilized support by activating said support by causing to take place on the support a reaction between a mercurous compound and agent capable of reacting with this mercurous compound, in the presence of water, to form metallic mercury, either by reduction or by disproportionating one of the reaction components being located in the support or at least in the surface thereof, and produce thereby an at least partly external, hardly visible layer of mercury germs on the support.

According to the invention, the hardly visible mercury germ layer is then treated with a non-stabilized noble metal intensifying bath comprising a non-stabilized aqueous solution of a metal compound more noble than copper and a reducing agent for the noble metal compound. By means of this step noble metal from the noble metal compound of this solution is reduced on the mercury germ layer and forms an electrically conductice noble metal layer having a resistance not greater than 10 ohms per square.

For the noble metal depositing bath there may be used the conventional baths for chemical silver deposition, but preferably use will be made of the purely physical developers usually intended for photographic purposes, since, in view of their great selectivity, they may be more easily controlled. Purely physical developers are distinguished from the so-called chemical silver deposition baths by the use of photographic reducing agents, i.e., compounds of which the reducing eifect in the physical developer under the prevailing conditions, owing to the presence of the photographic metal germ image, is accelerated so that a sufliciently selective noble-metal deposition on this metal germ image is obtained. A frequently used physical developer is, for example, a solution of silver nitrate in water, to which is added metol, hydroquinone or p-phenylene-diamine. Such a developer contains, furthermore, usually substances to improve the preservability thereof or to control the speed of development, for instance, organic acids, buffer mixtures or substances reacting with the noble-metal compound, while forming a complex.

By the term non-stabilized noble-metal intensifying bath in the instant case is meant a noble-metal depositing bath, as defined supra, free of ionic surfactants added to prevent the noble-metal compound from being spontaneously reduced to metal in the aqueous solution.

3,179,575 Patented Apr. 20, 1965 Although it is possible to include, in the support during the manufacture thereof, the agent capable of reacting with the mercurous compound, in the presence of water, the activation of the support is usually carried out by treating the support surface, in order of succession, with an aqueous solution of an agent capable of reacting with a mercurous compound, in the presence of water, while separating out metallic mercury, and with an aqueous solution of a mercurous compound or conversely.

In order to keep the mercury layer formed uniform, it is advisable to wipe off the surface of the support after the first treatment, in order to remove adhering fluids. Particularly in the case in which the treatment with the mercurous compound reacting agent precedes and the support thus treated is afterwards treated with mercurous solution, better results are obtained, when the layer is dried after the first treatment.

Since the present invention is concerned with the manufacture of external, electrically conductive noble-metal layers by the intensification of external mercury germs, which lies on the support and which are obtained during the activation of the support by a reaction between a mercurous compound and an agent capable of reacting with this mercurous compound in the presence of water, to separate mercury metal, it will be obvious that care must be taken, by choosing the ratio between the concentrations of the mercurous compound solutions and the solutions of the mercurous reacting compound, that the reaction takes place indeed mainly at the surface of the support. A partial deposition in or below the surface is conducive to the adhesion of the noble-metal layer to the support. Irrespective of the order of succession in the two bath treatments, there can always be indicated, as far as the solubilities of the substances concerned permit doing so, a range of concentrations of the second bath at a given concentration of the first bath to obtain an at least partly external layer of mercury germs on the support, on which is to be deposited subsequently an external noble-metal layer with a resistance of not more than 10 ohms per square by treatment with the nonstabilized noble-metal intensifying bath. The limits of the said range may differ slightly with the individual compounds and supports. Therefore, the active range of concentrations of the second bath, with a given concentration of the first bath, must be fixed by a simple sequence of experiments, while the concentrations of the second bath are varied by a given factor (for example 2 to 3). It has been found that, at least when the support is dried intermediately between the two treatments,

the concentration of the first bath must, as a rule, exceed that of the second bath. When the supportis not dried between the treatments, materially higher concentrations of the first bath are to be employed than in the case of drying.

Very good results are obtained, when the production of mercury from the mercurous compound is performed by means of a solution of a thiosulfate or a carbonate.

When the agent producing mercury from the mercurous compound is contained in the first bath, the method according to the invention can yield high-gloss, excellently reproduceable, external, electrically conductive noblemetal layers by adding to the second bath, together with the mercurous compound, one or more organic hydroxy acids, selected from the group of citric acid, tartaric acid, glycolic acid, glycerol acid and malic acid. In this case, such a quantity of the acid concerned is to be added, that an initially formed precipitate with the said mercurous compound is re-dissolved.

The method according to the invention may be employed for the manufacture of uninterrupted surfaces of noble-metal or non-metallic supports as Well as for the manufacture of noble-metal patterns thereon, such as used for decoration, electrical and electro-technical purposes. In the first case, the whole support surface is to be sub- 'jected to the activation. In the second case, the pattern may be applied previously by activating the surface in accordance with the pattern either directly by means of a printing technique or indirectly with the aid of a known mechanical or photographic masking technique, or the pattern may be obtained on the uninterrupted noble-metal layer finally obtained in accordance with the invention by etching away the redundant noble metal subsequent to covering by the noble-metal layer. All kinds of known methods of photo-engraving and graphics may be used in this case to apply a mask resisting the chemical treatment to be employed.

The external noble-metal layers obtained by the method according to the invention often have electric resistances which are many times higher than is calculated from the resistivity of the metal concerned in a compact state. By a thermal and/ or chemical after-treatment and/or a mechanical polishing treatment the excessively high resistance of the layer can be considerably reduced, which is desirable for various electrical uses.

It is known (B. Raub, Z. Metallkunde 39, 33 (1948)) that with the electro-deposition of, for instance, silver metal, under certain conditions, non-metallic substances may, in addition, be included in the layer as impurities which affect particularly the electric resistance and the hardness of the layer. The high resistance of such a layer can be reduced by a thermal treatment in excess of a given temperature so that it approaches that value which can be calculated from the resistivity of the compact, pure metal.

In accordance with the invention, it has been found that heating of the external noble-metal layer obtained by intensification of an external layer of mercury germs to a temperature of at least 80 C. may result in a reduction of an excessively high resistance value of the layer. This heating may be performed, for instance, in a furnace with the aid of an infrared radiator, by electric current passing through the layer or by means of rinsing in a hot, chemically indifferent liquid (water or glycerol, fo example). The desired effect is attained more rapidly as the temperature of the after-treatment is higher, but it must, of course, be considered that the material of the support constitutes a limit to the temperature that may be employed.

It has furthermore been found that the silver metal separated out in several, non-stabilized, purely physical developers has a more or less loose structure (so-called black silver), which can be rendered more compact by given chemical after-treatments (so-called grey silver). In this connection, reference is made to C. K. Mees, The Theory of the Photographic Process, revised edition, 1954, page 753. As may be expected, the electric resistance is materially reduced. The phenomenon may be due to the flocculation of sols known from colloid chemistry. Any important reduction in electric resistance of the external silver layer obtained subsequent to intensification may be obtained by contacting this layer with an aqueous solution of one or more compounds separating off a hydrogen ion or an anion potential being determining with respect to the silver metal, for example, the chloride, bromide, iodide, thiocyanate, cyanide, sulphide, sulphite, thiosulphate or hydroxyl ion. For compounds separating oif hydrogen or hydroxyl ions use may be made, not only of a diluted, non-oxidizing acid or a diluted base, but also of a solution of an acid or an alkali-reacting salt. As a rule, the resistance reducing effect is obtained more rapidly as the active compound is provided in a higher concentration, while with respect to the extent of the activity of the various anions the following sequence may be established:

As an alternative, the said reduction in resistance may be obtained by mechanical polishing.

For a great number of uses in the electrical and the electro-technical and the decoration fields it is necessary or desirable that the external, electrically conductive noblemetal layer obtained by the method according to the invention should be electro-chemically after-treated, which may be followed by a superficial chemical conversion or coloration.

To this end may be used, for example, electrolytic polishing, electrolytic deposition of metals, while using or not using an external current source, electrolytic coloration of the deposited metal or electrophoretic coating with a protective or insulating or photo-conductive and/or semi-conductive surface layer.

Moreover, a combination of various electro-chemical and/ or chemical after-treatments is possible.

When the invention is to be used for the manufacture of metal patterns which are electrolytically intensified or electrophoretically coated the portions of the uninterrupted noble-metal layer not associated with the final pattern will be provided, preferably prior to the required treatment, with a resistant mask with the aid of a mechanical or photographic technique. The uncovered portions of the layer are then intensified or coated, while the uninterrupted layer provides a conductive contact between the parts of the final pattern. After removal of the mask, the pattern may be produced by etching away the redundant noble metal. The mask may, of course, also be applied subsequent to the electro-chemical after-treatment, the pattern being then obtained by etching away the metal of the uncovered portions of the layer. However, this is economically less advantageous. For the various uses there is furthermore a great choice among a number of known, mechanical after-treatments, which may be combined with the described after-treatments. A few, important, suitable after-treatments of this kind are, inter alia: mechanical polishing of the surface of the layer, application of a lacquer or varnish layer to the layer surface, embedding of the layer together with the support in an insulating envelope of thermo-hardening or thermoplastic material, transfer of the layer, if desired together with the support, to a diflerent support of high electric qualities, also of thermo-hardening or thermo-plastic material, application of electric connections by soldering (for example, dip soldering).

In electronics, use may, for example, be made of external noble-metal layers manufactured in accordance with the invention in conjunction with one or more of the aforesaid aftertreatments for the manufacture of printed wirings, printed circuits, screening grids, switches and other component parts.

Substantially, all non-metallic, electrically nonconductive support materials, which are, as such, not quite accessible for the various baths for after-treatment may, fre quently with the aid of known methods, be rendered hy drophilic to some extent, at least superficially, which suffices, either by given chemical surface treatments or by applying a thin, slightly hydrophilic coating. For example, polystyrene, polyacrylate, bakelite and various other high-polymer products may be rendered accessible for a surface treatment with chlorosulphonic acid (T. Westermark, Acts. Chem. Scand. 6, 1194-1199 (1952)). Cellulose esters, polyvinyl acetate and -chloride and so on may be rendered suitable by superficial saponification; glass may be provided by a chemical surface treatment or by coating with a thin layer of silica gel (F. L. Burmistrow, Phot. Journ. 76, 452-459, 1936). Gelatin, regenerated cellulose, paper, wood, polyvinyl alcohol and so on may be used without the need for further means.

If a slightly hydrophylic coating is used, the mercurous compound or the agent capable of reacting with the mercurous compound in the presence of water while separating out mercury can be absorbed directly therein.

EXAMPLE I Strips of superficially saponified cellulose tri-acetate foil were impregnated for two minutes in aqueous solutions of the agents indicated in the first column of Table I, all of which are capable of reacting with a mercurous compound in the presence of water, while separating out metallic mercury. The employed concentrations of these substances in the impregnation liquids are also indicated in the said table. The strips are then wiped off and dried in air at room temperature with the exception of the strips impregnated with sodium sulphite, which are dried in a nitrogen atmosphere. Then the strips are dipped, for a few seconds, in an aqueous solution containing, per litre, 0.005 mol of mercurous nitrate and 0.005 mol of nitric acid. Then they were washed, for sec., in distilled water and, for 10 min., treated in an intensifying bath containing 0.5 g. of metol, 2 g. of citric acid and 0.2 g. of silver nitrate in 100 g. of distilled water. Finally, the strips were again washed in water and dried in air. In Table I a sign -I indicates that external, electrically conductive silver layers were obtained on the strips with an electric resistance of not more than 10 ohm per square. The sign indicates that no external, electrically conductive silver layers according to the invention were obtained, so that the concentrations concerned cannot be used for the activation of the strips.

Instead of using the indicated solution for the intensification, but by using one of the following, physical developers containing per 100 g. of distilled water:

(a) 0.5 g. of metol, 2 g. of lactic acid, 0.1 g. of silver nitrate (developing time 10 min.),

(b) 1 g. of hydroquinone, 0.2 g. of citric acid and 0.1

g. of silver nitrate (developing time min.),

(0) 1g. of hydroquinone, 0.1 g. of lactic acid and 0.1

' g. of silver nitrate (developing time 12 min.),

(d) 0.5 g. of pyrogallol, l g. of citric acid, 0.1 g. of

silver nitrate (developing time 15 min.),

or a silver-depositing solution obtained by mixing parts by volume of the solution 1, 5 parts by volume of the solution 2 and 1.1 parts by volume of the solution 3, d

which solutions have the following compositions.

Solution 1: 1 g. of silver nitrate, 2.5 g. of ammonium nitrate, 100 g. of distilled water,

Solution 2: 1.6 g. of sodium hydroxide and 100 g. of

distilled water,

Solution 3: 1.25 g. of hydrazine sulphate and 100 g. of

distilled water;

qualitatively corresponding results are obtained.

EXAMPLE n In the manner described in Example I (strips of the support material referred to therein were treated with aqueous solutions of sodium thiosulphate with the concentrations indicated in Table II. After wiping off and drying, the strips were immersed, for a few seconds, into one of the mercurous nitrate solutions with the concentrations indicated in the Table. These soltuions were produced by diluting with distilled water a standard solution containing, per litre, 0.05 mol of mercurous nitrate and 0.05

Concentration Nagsgos in mol/litre C0110. HgflNOg):

- mol/litre llll An addition of, for example, citric acid to the mercurous nitrate solution for example 0.1 mol to the solution containing the 0.005 mol of mercurous nitrate and.0.005 mol of nitric acid per litre results in an improved gloss of the final silver layer.

EXAMPLE 111 Table III relates to strips of a superficially saponified cellulose tri-acetate foil treated in the manner described in Example I with the exception that, after impregnation in the solution of the agents capable of-reacting with a mercurous compound in the presence of water, while separating out metallic mercury, the strips were wiped off but dipped Without intermediate drying in the solution of mercurous nitrate. The signs of Table III have the same meanings as those of the preceding Example. A comparison of results of Table III with those of Table I shows that the omission of the drying process results in that, as a rule, materially higher concentrations ofpthe first activating solution are required than in the case of drying.

Table III EXAMPLE IV Strips of the support material of Example I were dipped for two minutes in an aqueous solution containing per litre 0.005 mol of mercurous nitrate and 0.005 mol of nitric acid. Immediately after the strips had been wiped ofif, they were treated, for two seconds, with solutions of sodium thiosulphate with the concentrations indicated in Table IV. After washing in distilled Water, the strips were treated with metol citric acid developer of Example I.

Strips of the support material of Example I were dipped, for two minutes, in aqueous solutions containing, per litre, 0.0040.008-0.0l6 and 0.032 mol of sodium carbonate and sodium thiosulphate respectively. After wiping off and drying the rear side of the support was covered with a lacquer layer. Subsequently, the strips were dipped, for a few seconds, in the mercurous nitrate solution of Example IV. After washing in distilled water, for a short time, the strips were intensified, for 10 minutes, .in a physical developer obtained by dissolving in distilled water: metol (0.025 mol/litre), citric acid (0.1 mol/litre) and silver nitrate (0.01 mol/litre).

Finally the strips were washed in water and dried in air. The electric resistance values of the external silver layers on the strips activated with the aid of sodium carbonate varied between 130 and 500 ohms per square. After a thermal treatment at 150 C. for one hour, these values had dropped to about 0.25 ohm per square. The resistance values of. the silver layers on the strips activated with the aid of sodium thiosulphate varied between 170 and 1100 ohms per square. These values dropped by the said thermal treatment of 0.250.6 ohm per square.

EXAMPLE VI Strips of the support material of Example I were dipped for two minutes in one of the following aqueous solutions:

(A) Thiourea (0.1 mol/litre), (B) Sodium thiosulphate (0.03 mol/litre), (C) Potassium thiocyanate (0.05 mol/litre).

After wiping oif and drying in open air the strips were dipped, for a few seconds, in the mercurous nitrate-nitric acid solution of Example IV. After washing in distilled water, the strips treated with the solutions (A) and (B) were intensified in the physical developer of Example V, whereas the strip treated with the solution (C) was intensified in the metol-citric acid developer of Example I. The developing time was, in all cases, 10 minutes. Finally, the strips were washed in water and dried. The external silver layer on the strip obtained with the solution (A) had an electric resistance of 160 ohms per square, which dropped to 0.4 ohm per square after a treatment at 120 C. for two hours. The silver layer on the strip treated with solution (B) had a resistance of 180 ohms per square which dropped to 0.25 ohm per square by a chemical after-treatment for 2 minutes with a solution of potassium chloride (0.1 mol per litre). By using solutions of sodium bromide, sodium iodide, sodium thiosulphate, sodium sulphite, sulphuric acid, potassium hydroxide, potassium thiocyanate or sodium sulphide instead of the KCl solution material reductions in the original resistance values may be obtained. The silver layer on the strip treated with solution (C) had a resistance of 180 ohms per square. This value was reduced to 13 ohms per square by rubbing with a plug of cottonwool. A further piece of the layer was subjected to a thermal after-treatment at 150 C. for one hour. Then the resistance amounted to 0.7 ohm per square.

EXAMPLE VII An external silver layer on cellulose tri-acetate, obtained in the manner described in Example VI by activation with thiourea and mercurous nitrate, was electrolytically intensified with a few microns of copper by using an aqueous copper-deposition bath of the following composition:

Copper sulphate (SH O) (200 g. per litre), Concentrated sulphuric acid (50 g. per litre), Phenolsulphonic acid sodium (0.5 g. per litre).

The current density was 5 a./dm. and the time of copper deposition was 2 min. This copper layer was then covered electrophoretically with a layer of aluminum oxide by using a suspension of of alundum in methanol with a field intensity of 37.5 v./ cm. for 6 sec. and a current of 70 ma.

EXAMPLE VIII layer was washed in water and dried in air. The electric resistance of the silver layer amounted to 500 ohms per square, which value dropped to 7 ohms per square, after a treatment at C. for one hour.

EXAMPLE IX On the support material of Example VIII, there was stamped the desired pattern with the aid of a rubber stamp. The rubber stamp was wetted by means of an inking pad with a 0.05 mol solution of sodium carbonate in water. The sodium carbonate solution had been rendered slightly viscous with the aid of a thickening agent. The activation and the intensification of the mercury germ layer took place in the manner described in Example VIII. By this means there Was obtained on the paper an external, electrically conductive silver pattern.

EXAMPLE X An external silver layer formed on the support material of Example I in the manner described in Example VIII was covered with a uniform layer of Kodak Photoresist by pouring it out or spraying it. After drying in air, the light-sensitive layer was exposed, for 5 min. behind a pattern (line grid) with the aid of a high-pressure mercury-vapour lamp of 500 w. at a distance of about 50 c.m.s. After the exposure, the layer was developed with Kodak Photoresist Developer or with trichloroethylene. Then, the mask thus obtained was hardened by treating it at 150 C. for about 10 min. The uncovered portions of the silver layer were then plated with copper for 15 minutes with a current density of 4 a./dm. in an elctrolytic bath containing 20% by weight of copper sulphate (5H O) and 6% by weight of sulphuric acid in distilled water. After the mask was removed, the non-plated silver layer was removed by etching. If desired, the metal layer may finally be provided with a protective lacquer layer or with a protective layer of thermo-hardening or thermo-plastic material. In this manner, screening grids may be manufactured.

While we have described our invention in connection With specific embodiments and applications, other modifications thereof will be readily apparent to those skilled in this art without departing from the spirit and scope of the invention as defined in the appended claims.

What is claimed is:

1. A method of producing a permanently adhering external, electrically conductive silver layer on a non-metallic, electrically non-conductive support, at least the surface of said support being hydrophilic, comprising the steps of applying to said support a mercurous compound, water and a compound capable of converting said mercurous compound in the presence of water to free metallic mercury thereby forming a barely visible, at least partially external germ layer of mercury metal on said support, said compound capable of converting said mercurous compound being selected from the group consisting of compounds that reduce mercurous compounds to free mercury metal and compounds that form free mercury metal from mercurous compounds by a disproportionating reaction, said mercury germ layer extending at least partially into the hydrophilic portion of said support, and then treating said mercury germ layer on said support with a silver metal intensifying bath, said bath comprising an aqueous solution of a water soluble silver compound and a water soluble reducing agent for said silver compound to thereby cause a visible, permanently adhering, electrically conducting silver layer to be deposited on said mercury germ layer on said support.

2. The method of claim 1 wherein the mercury germ layer is formed by treating the support in succession first mercury metal.

4. The method of claim 1 wherein the compound capable of converting the mercurous compound is a compound which converts the mercurous compound into a mercuric compound and free mercury metal by a disproportionating reaction.

5. The method of claim 4 wherein the compound capable of converting the mercurous compound is introduced into the support surface prior to the addition of an aqueous solution of the mercurous compound and this aqueous solution contains, in addition to the mercurous compound an organic hydroxy acid selected from the group consisting of citric acid, tartaric acid, glycolic acid, glycerol acid and malic acid.

6. The method of claim 1, wherein the resultant silver layer is treated with an aqueous solution of at least one water soluble compound yielding in solution an ion selected from the group consisting ofthe chloride, bromide, iodide, thiocyanate, cyanide, sulfide, sulfite, thiosulfide, hydroxyl and hydrogen ions.

7. The method of claim 6, wherein the ion yielding compound is selected from the group consisting of nonoxidizing acids and acid salts.

lid

8. The method of claim 1 wherein the electrical conductivity of the silver layer is increased by heating said layer to a temperature of at least 80 C.

9. Themethod of claim 1 wherein the electrical conductivity of the silver layer is increased by the electrolytic deposition of additional metal on said silver layer.

10. The method of claim 1 wherein the electrical conductivity of the silver layer is increased by mechanically polishing said silver layer.

References Eited by the Examiner UNITED STATES PATENTS 168,442 10/75 Worthen an 204-20 641,709 1/00 Legato 204- 33 2,195,231 3/40 Weder 204-40 2,906,682 9/59 Fahnoe 61:61 204-481 2,940,013 6/60 L66 204-15 FOREIGN PATENTS 509,965 7/39 Great Britain.

JOHN H. MACK, Primary Examiner. JGSEPH REBOLD, Examiner. 

1. A METHOD OF PRODUCING A PERMANENTLY ADHERING EXTERNAL, ELECTRICALLY CONDUCTIVE SILVER LAYER ON A NON-METALLIC, ELECTRICALLY NON-CONDUCTIVE SUPPORT, AT LEAST THE SURFACE OF SAID SUPPORT BEING HYDROPHILLIC, COMPRISING THE STEPS OF APPLYING TO SAID SUPPORT A MERCUROUS COMPOUND, WATER AND A COMPOUND CAPABLE OF CONVERTING SAID MERCUROUS COMPOUND IN THE PRESENCE OF WATER TO FREE METALLIC MERCURY THEREBY FORMING A BARELY VISIBLE, AT LEAST PARTIALLY EXTERNAL GERM LAYER OF MERCURY METAL ON SAID SUPPORT, SAID COMPOUND CAPABLE OF CONVERTING SAID MERECUROUS COMPOUND BEING SELECTED FROM THE GROUP CONSISTING OF COMPOUNDS THAT REDUCE MERCUROUS COMPOUNDS TO FREE MERCURY METAL AND COMPOUNDS THAT FORM FREE MERCURY METAL FROM MERCUROUS COMPOUNDS BY A DISPROPORTIONATING REACTION, SAID MERCURY GERM LAYER EXTENDING AT LEAST PARTIALLY INTO THE HYDROPHILIC PORTION OF SAID SUPPORT, AND THEN TREATING SAID MERCURY GERM LAYER ON SAID SUPPORT WITH A SILVER METAL INTENSIFYING BATH, SAID BATH COMPRISING AN AQUEOUS SOLUTION OF A WATER SOLUBLE SILVER COMPOUND AND A WATER SOLUBLE REDUCING AGENT FOR SAID SILVER COMPOUND TO THEREBY CAUSE A VISIBLE, PERMANENTLY ADHERING, ELECTRICALLY CONDUCTING SILVER LAYER TO BE DEPOSITED ON SAID MERCURY GERM LAYER ON SAID SUPPORT. 